Distribution Behaviors Of Impurity Elements And Inclusions In DP And MS High-strength Steels And Their Effects On Properties

At present.the industrial control of P content in advanced high-strength steel is very strict,resulting in a significant increase in the rejection rate and production costs.The study of trace P in advanced high-strength steel has not been reported.Compact strip production(CSP)is a new type of short process steel manufacturing technology with low manufacturing cost and high production efficiency.Studies on CSP mainly focus on the positive effects of alloying elements and their relationship with microstructure and properties.However,there are no reports on the negative effects of impurities and inclusions in the high strength martensitic steel produced by CSP process.In this paper,the distribution behavior of impurity element P in the Ti-Mo and Ti-Nb microalloyed DP and its influence on mechanical properties were systematically studied by means of FactSage,first-principles calculation,3D-APT and other advanced analytical characterization methods.The distribution characteristics and process evolution of inclusions in the martensitic steel produced by CSP process and their effects on properties were systematically studied.The following innovative research results have been achieved:(1)The existing forms of impurity elements P,S and N,and the influence by alloy elements were calculated by FactSage.The results show that P mainly exists in the form of solid solution,while S and N exist in the form of precipitates such as MnS.CaS and TiN.The first-principles calculation results show that for three types of MC carbides(TiC,(Ti,Mo)C and(Ti,Nb)C).P and S atoms tend to segregate into the α-Fe(001)/MC(001)interface corresponding to the nearest neighbor of the Fe and M atoms.(2)The 3D-APT results of cold-rolled annealing plate of Ti-Mo microalloyed DP steel show the impurity P is mainly distributed in ferrite and martensite matrix.and a small amount of P is distributed around(Ti.Mo)C.The concentration of solid solution P in ferrite(0.022 at.%)is slightly less than that in martensite(0.025 at.%).The maximum concentration of P atom adsorbed by(Ti,Mo)C at the ferrite and martensite matrix and ferrite/martensite(F/M)phase boundary are 1.67 at.%,1.14 at.%and 0.59 at.%,respectively.(Ti,Mo)C can capture P more effectively than F/M phase boundary and eliminate P segregation at F/M phase boundary.The tensile properties,conventional impact toughness,and impact toughness of precharged hydrogen and prestrain specimens of DP steel are not significantly affected by P variation in the range of trace(≤0.015%).(3)The impurity P in the cold-rolled annealed sheet of Ti-Nb microalloyed DP steel is mainly dissolved in the martensite matrix,and a small amount of P segregates into the(Ti.Nb)C and the prior austenite grain boundary.When P content is ≤0.015%,there is no significant difference in microstructure,tensile properties and impact properties between the two DP steels.When the content of P increases to 0.022%.the microstructure and impact toughness of DP steel have no significant change,but the strength increases and the total elongation decreases slightly.When P content is further increased to 0.12%,the equivalent diameter of(Ti,Nb)C carbide decreases significantly and its number increases,the effective grain size of martensite is refined,the yield strength and tensile strength of DP steel increase significantly,the elongation does not decrease,but the impact toughness decreases.The P segregation band appeared in 0.12P steel,when it was impacted at-80℃,brittle intergranular fracture occurred in the local area of the fracture surface.(4)The harm of P.to DP steel can be further reduced by adjusting the microstructure of Ti-Nb microalloyed 0.022P steel.With the increase of annealing temperature,the martensite volume fraction of 0.022P steel increases,the ferrite grain size decreases,the strength and impact toughness of DP steel increase,and the elongation decreases first and then increases.The yield strength of 820T steel increases by 13.1%,and the tensile strength increases by 5.7%compared with 0.006P steel,while the elongation and impact toughness do not decrease.The morphology and volume fraction of martensite in DP steel have significant effects on its mechanical properties.With the increase of martensite volume fraction,the strength increases and the total elongation decreases;The increase of martensite volume fraction decreases the impact toughness of DP steel at room temperature,but increases its impact toughness at low temperature.For DP steel mainly composed of martensite,when the martensite C content is the same,the size of martensitic block decreases,the martensite toughness increases,and the impact toughness of DP steel increases;when martensite C content is high,martensitic twins significantly reduce the low-temperature impact toughness of DP steel.(5)The types of inclusions in martensitic steel in CSP short flow and conventional flow are the same,which are Al2O3-CaO,Al2O3-CaO-CaS,CaS-MnS and(Ti,Nb)(C,N).The inclusions in the two martensitic steel billets are mainly Al2O3-CaO-CaS and CaS-MnS.but the total amount of CSP-Slab inclusions is large and its size is small.After hot rolling,the total amount of CSP-HR inclusions decreased slightly,and the fraction of large size and high aspect ratio Al2O3-CaOCaS inclusions increased.After cold rolling and annealing,the proportion of small size Al2O3-CaO-CaS and large aspect ratio CaS-MnS in CSP-MS steel increases significantly,leading to an increase in the total number of inclusions.The inclusions in CSP-MS steel are mainly spherical Al2O3-CaO CaS,slender MnS and smallsized TiN.The inclusions in Con-MS steel are mainly large-sized TiN,spherical Al2O2—CaO-CaS and small-sized MnS.(6)The strength of annealed cold-rolled sheets of martensitic steel in the two processes is close,the elongation of CSP-MS is slightly less than that of Con-MS,and the average length and aspect ratio of MnS in CSP-MS steel is greater than that of Con-MS.which results in the mechanical properties,especially the impact toughness,anisotropy being stronger than that of Con-MS.Aspect anisotropy of MnS inclusions leads to different fracture stresses in different directions.When the fracture direction is perpendicular to RD,MnS microcracks are not easy to propagate into the matrix.When the fracture direction is parallel to RD,MnS microcracks are easy to grow and polymerize,and to extend into the matrix.TiN microcracks are sharp and easy to propagate into the matrix,while the microcracks caused by interface debonding between the spherical Al2O3-CaO-CaS inclusion and the matrix is smooth and not easy to extend to the matrix.The TiN inclusions of CSP-MS steel are small in size and less in amount,resulting in its better resistance to hydrogen-induced delayed cracking than Con-MS steel.